nVMX: Skip EPT tests if INVEPT(SINGLE_CONTEXT) is unsupported

EPT can technically be supported without INVEPT(SINGLE_CONTEXT), skip the
EPT tests if SINGLE_CONTEXT isn't supported as it's heavily used (without
the result being checked, yay).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-22-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>